The present invention disclosed herein relates to a secondary battery and a cathode and a cathode active material used therein.
Recently, lithium secondary batteries have been used in various fields including portable electronic devices such as mobile phones, personal digital assistants (PDAs), and laptop computers. In particular, in line with growing concerns about environmental issues, research into lithium secondary batteries having high energy density and discharge voltage as a power source of an electric vehicle able to replace vehicles using fossil fuels such as gasoline vehicle and diesel vehicle, one of major causes of air pollution, have been actively conducted and some of the research are in a commercialization stage. Meanwhile, in order to use a lithium secondary battery as a power source of the electric vehicle, the lithium secondary battery must maintain stable power in a usable state of charge (SOC) range along with high power.
An electric vehicle is classified as a typical electric vehicle (EV), battery electric vehicle (BEV), hybrid electric vehicle (HEV), or plug-in hybrid electric vehicle (PHEV) according to a power source thereof.
The HEV among the foregoing electric vehicles is a vehicle obtaining a driving force from the combination of typical internal combustion engine (engine) and electric battery, and has a mode, in which the driving force is mainly obtained through the engine while the battery assists insufficient power of the engine only in the case of requiring more power than that of a typical case, such as uphill driving, and SOC is recovered again through charging the battery during stop of the vehicle. That is, the engine is a primary power source in the HEV, and the battery is an auxiliary power source and is only used intermittently.
The PHEV is a vehicle obtaining a driving force from the combination of engine and battery rechargeable by being connected to an external power supply, and is broadly classified as parallel-type PHEV and series-type PHEV.
In the parallel-type PHEV, the engine and the battery are in an equivalent relationship to each other as a power source and the engine or the battery may alternatingly act as a primary power source according to the situation. That is, the parallel-type PHEV is operated in a mutually parallel mode, in which the battery makes up for insufficient power of the engine when the engine becomes a primary power source and the engine makes up for insufficient power of the battery when the battery becomes a primary power source.
However, the series-type PHEV is a vehicle basically driven only by a battery, in which an engine only acts to charge the battery. Therefore, since the series-type PHEV entirely depends on the battery rather than the engine in terms of driving of the vehicle, differing from the HEV or the parallel-type PHEV, maintaining of stable power according to battery characteristics in a usable SOC range becomes a very important factor for driving safety in comparison to other types of electric vehicles. The EV also requires a battery having a wide available SOC range.
Meanwhile, with respect to LiCoO2, a typical cathode material of a high-capacity lithium secondary battery, practical limits of an increase in energy density and power characteristics have been reached. In particular, when LiCoO2 is used in high energy density applications, oxygen in a structure of LiCoO2 is discharged along with structural degeneration in a high-temperature charged state due to its structural instability to generate an exothermic reaction with an electrolyte in a battery and thus it becomes a main cause of battery explosion. In order to improve the safety limitation of LiCoO2, uses of lithium-containing manganese oxides, such as layered crystal structure LiMnO2 and spinel crystal structure LiMn2O4, and lithium-containing nickel oxide (LiNiO2) have been considered, and a great deal of research into layered structure lithium manganese oxides expressed as the following Chemical Formula 1, in which Mn as an essential transition metal is added in an amount larger than those of other transition metals (excluding lithium) to layered structure lithium manganese oxide as a high-capacity material, has recently been conducted.aLi2MnO3·(1−a)LixMO2  [Chemical Formula 1]
(where 0<a<1, 0.9≦x≦1.2, and M is any one element or two or more elements selected from the group consisting of aluminum (Al), magnesium (Mg), manganese (Mn), nickel (Ni), cobalt (Co), chromium (Cr), vanadium (V), and iron (Fe).)
The lithium manganese oxide exhibits relatively large capacity and also exhibits relatively high power characteristics in a high SOC range. However, resistance may rapidly increase at an operating voltage limit, i.e., a low SOC range, and thus power may rapidly decrease and initial irreversible capacity may be large.
Various explanations related thereto have been described, but this may be generally described below. That is, the reason for this is that, as shown in the following reaction formulas, two lithium ions and two electrons are eliminated along with oxygen gas from Li2MnO3 constituting the layered structure lithium manganese oxide composite in a high voltage state of 4.5 V or more based on cathode potential during initial charge, but one lithium ion and one electron are only reversibly inserted into a cathode during discharge.
(Charge) Li2Mn4+O3→2Li++2e−+½O2+Mn4+O2 
(Discharge) Mn4−O2+Li++e−→LiMn3+O2 
Thus, initial charge and discharge efficiency of aLi2MnO3·(1−a)LiMO2 (0<a<1, M=Co, Ni, Mn, etc.) may differ according to a content of Li2MnO3 (a value), but may be lower than a typical layered structure cathode material, e.g., LiCoO2, LiMn0.5Ni0.5O2, or LiMn0.33Ni0.33Co0.33O2.
In this case, since capacity of an anode must be over-designed in order to prevent lithium precipitation at the anode during an initial cycle according to the large irreversible capacity of aLi2MnO3·(1−a)LiMO2, actual reversible capacity may decrease. Accordingly, efforts have been made to control such irreversible characteristics by using surface coating or the like, but limitations such as productivity may not be completely resolved to date. Also, with respect to the layered structure material, some limitations in safety have been reported.
Since there are disadvantages and limitations in using typical cathode active materials of a lithium secondary battery alone, use of a mixture formed thereof is required. In particular, in order to be used as a power source of medium and large sized devices, there is an urgent need for a lithium secondary battery having safety improved by exhibiting a uniform profile without a rapid voltage drop in an entire SOC range as well as having high capacity.